The phr (parts per hundred parts of rubber by weight) data used in this specification are the conventional quantitative data used in the rubber industry for mixture formulations. The amount added in parts by weight of the individual substances here is always based on 100 parts by weight of the entire composition of all of the rubbers present in the mixture.
Pneumatic tires are strengthened by textile or metallic reinforcement, e.g. with brass-coated steel cord, in order to withstand high mechanical stresses. Pneumatic tires comprise by way of example brass-coated steel cord in the belt, in the bead core, and optionally in the carcass. In order to ensure that the rubber-reinforcement composite is durable, the embedding rubber mixture (rubberizing mixture) is intended to exhibit good adhesion to the reinforcement, and this adhesion should not be impaired by aging and by storage in moist conditions. The vulcanizates should moreover exhibit high dynamic and mechanical strength and low susceptibility to cracking and to crack propagation.
The adhesion of rubber to textile reinforcement is achieved by way of impregnation (e.g. with resorcinol-formaldehyde resins in combination with rubber latices (RFL dip)) by the direct method using adhesive mixtures or by way of adhesive solutions of unvulcanized rubber using polyisocyanates.
The rubber-metal adhesion can be advantageously influenced by use of what are known as reinforcing resins in the rubberizing mixture. Examples of known reinforcing resins are lignin, polymer resins, and phenol-formaldehyde resins with hardener. A method that has long been known for improving the rubber-metal adhesion is to use cobalt salts and/or a resorcinol-formaldehyde-silica system, or a resorcinol-formaldehyde system as additions for the rubberizing mixtures. Rubberizing mixtures with cobalt salts and with a resorcinol-formaldehyde-silica system are known by way of example from KGK Kautschuk Gummi Kunststoffe No. 5/99, pp. 322 to 328, from GAK 8/1995, p. 536, and from U.S. Pat. No. 7,307,116.
Fillers used in known rubberizing mixtures are carbon black and/or silica in the following carbon-black-to-silica ratios: from 100:0 to 80:20, or else from 20:80 to 0:100.
Pneumatic tires comprising sulfur-crosslinked rubber mixtures are known from U.S. Pat. Nos. 5,871,592 and 6,169,137. In those documents, a mixture with natural rubber, silica, silane coupling agent and adhesive system is used as rubberizing mixture for the belt, where mixtures for rubberizing the belt are subject to quite different requirements for example in respect of heat accumulation than the requirements applicable by way of example to carcass rubber mixtures, which usually have to have high resistance to fatigue, so that in the region of the side wall they withstand the deflection of the tire. The silica used is by way of example a high-dispersibility silica. The intention is to achieve a reduction in the rolling resistance of the tire, without any adverse effect on the other properties, such as adhesion, wear resistance, or durability in particular of the belt, or on the production of the tire. The expression high-dispersibility silica here means a silica which can break down or deagglomerate and which therefore can be dispersed (distributed) particularly well and uniformly in an elastomer matrix. This particularly good distribution can be demonstrated via electron micrographs or optical micrographs of thin sections. The silicas Ultrasil® VN2 and Ultrasil® VN3 marketed by Evonik Industries are not high-dispersibility silicas. The use of high-dispersibility silica is intended to reduce materials fatigue to a minimum in the vulcanized mixture and thus reduce the risk of separation of the mixture from reinforcement. The CTAB index of the high-dispersibility silicas used in U.S. Pat. Nos. 5,871,597 and 6,169,137 for belt mixtures is ≦125 m2/g.